Motor Minimum OCPD For Commercial Applications

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Aleman said:
In my humble opinion it isn't smart to protect a 1HP motor with a 20A breaker. 15 is better than 20 but still a little large.
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petersonra said:
Why do you think that?
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Aleman said:
Because I've replaced a lot of burned up motors.
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GoldDigger said:
The motor should be protected from burning up by a properly sized overload, not by a branch circuit breaker.
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ActionDave said:
That's not the breaker's fault.
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Perhaps not, but let's get back on track here and take that a few degrees of exaggeration higher to see what y'all have to say: same motor and properly overload protected on a 100A breaker?
 
GoldDigger said:
The motor should be protected from burning up by a properly sized overload, not by a branch circuit breaker.
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Took 21 posts before someone mentioned it may need more then just a standard circuit breaker to properly protect the motor.

DJR, if you haven't left yet there is two types of protection needed for motor circuits, and they are seldom provided by a single device.

(1) Short circuit and ground fault protection, which can be 250% of motor full load current with inverse time breakers, or even higher if that doesn't allow the motor to start, or can be 800% if using an instantaneous trip breaker, or more if that doesn't allow the motor to start.

(2) motor overload protection, which is sized specifically to the motor applied. It will have a slow response to the rush of high current that occurs during motor starting, but will be designed to monitor more closely the conditions of heating caused by the current drawn by the motor. Say you do have a motor rated at 8 amps but you do put it on the 20 amp breaker, what is going to protect that motor from overload should it be loaded to 14 amps continously? The 20 amp breaker is not going to trip, the motor however is pulling 175% of it's rated load - it will overheat and will be damaged if there is no overload protection to limit it to it's rating. Even the 15 amp breaker doesn't protect it from overload in this application.

Now many single phase motors already have internal overload protection - if that is what you have then all you need to worry about is the short circuit and ground fault protection. Most cases the 15 amp breaker probably holds during starting, if not then the 20 is likely necessary.

Rules (where they exist) that don't allow 15 amp breakers is usually only for general purpose 120 volt receptacles. For fractional HP motors on a three phase system (208, 240, 480 volts doesn't really matter, they all draw well under 15 amps) a 15 amp breaker is much higher setting then really necessary, and if it is the only overcurrent protection provided, you will destroy every motor on every overload condition it is exposed to.
 
Aleman said:
True but the OP didn't mention any overload, only the breaker.
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He also failed to provide further information when asked for it, well sort of. Did turn the topic of discussion towards general purpose receptacle outlets to some extent, yet the example he gave but did not confirm was his application in OP was for a 1 hp 480 volt three phase motor. If it were a 120 volt 1 hp single phase motor we maybe are agreeing that 20 amp breaker is fine, maybe even necessary. Reality is 15 amps is overkill for a 1 HP 480 volt three phase motor, yet would be allowed by code.

Instead of getting P-O'd and leaving maybe he needs to listen to what is being asked for further clarification, it is hard to answer correctly when you don't know what is being asked.

If he does have a 1 hp 480 volt three phase motor, I don't think NEC allows a 20 amp breaker as a general rule. Next higher standard size circuit breaker for that application would be 15 amps, next higher standard size time delay fuse would be 6 amps. If his AHJ will not allow anything less then 20 - I think only that AHJ can be reasoned with on why? As I said before though most of those 20 amp minimum rules where they exist only apply to general purpose receptacle circuits and not to all circuits.
 
Smart $ said:
Perhaps not, but let's get back on track here and take that a few degrees of exaggeration higher to see what y'all have to say: same motor and properly overload protected on a 100A breaker?
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No problem with that. I like these Allen Bradley 140M Motor protective circuit breakers. They have both thermal and magnetic trips.

$_35.JPG
 
Aleman said:
No problem with that. I like these Allen Bradley 140M Motor protective circuit breakers. They have both thermal and magnetic trips.

$_35.JPG
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but that would not leave us with 100 amp setting for the motor short circuit and ground fault protection if it were installed on a 100 amp feeder, it essentially becomes the branch circuit protective device, with overload protection set per the dial and a magnetic trip setting that is fixed - but is probably much less then 100 amps if the device is properly selected for said 1 HP 480 volt three phase motor.
 
Smart $ said:
Perhaps not, but let's get back on track here and take that a few degrees of exaggeration higher to see what y'all have to say: same motor and properly overload protected on a 100A breaker?
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woudn't 430.52 still require a maximum setting of 250% of motor FLA for inverse time breaker or 800% for instantaneous time breaker? Only if those values won't let the motor start are you permitted to go higher? If OP's motor is 1 hp 480V 3 phase, 100 amps is well above those figures.
 
kwired said:
woudn't 430.52 still require a maximum setting of 250% of motor FLA for inverse time breaker or 800% for instantaneous time breaker? Only if those values won't let the motor start are you permitted to go higher? If OP's motor is 1 hp 480V 3 phase, 100 amps is well above those figures.
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Yes. And I was attempting to get other participants back on this track.

It simply comes down to, the NEC permits a 15A [typical thermal-magnetic panelboard] breaker only because it is the smallest standard breaker rating. Only on the very rare occasion where the motor of topic trips the breaker on start up would one be permitted to go to a 20A breaker.
 
Smart $ said:
Yes. And I was attempting to get other participants back on this track.

It simply comes down to, the NEC permits a 15A [typical thermal-magnetic panelboard] breaker only because it is the smallest standard breaker rating. Only on the very rare occasion where the motor of topic trips the breaker on start up would one be permitted to go to a 20A breaker.
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Were you really setting us up or was that a clever bail out?:)
 
kwired said:
but that would not leave us with 100 amp setting for the motor short circuit and ground fault protection if it were installed on a 100 amp feeder, it essentially becomes the branch circuit protective device, with overload protection set per the dial and a magnetic trip setting that is fixed - but is probably much less then 100 amps if the device is properly selected for said 1 HP 480 volt three phase motor.
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I agree you shouldn't have a 100 amp circuit feeding a 1HP motor with only an overload. I need to read more closely before posting. You guys should have been lawyers:).
 
Aleman said:
I agree you shouldn't have a 100 amp circuit feeding a 1HP motor with only an overload. I need to read more closely before posting. You guys should have been lawyers:).
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I don't know about lawyers, but you hopefully learn when inspectors catch what may seem like minor details and you then have a mindset to look for such details.
 
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